This invention relates to the fabrication of semiconductor devices, and more particularly, but not exclusively, to a method of fabricating semiconductor wafers wherein spin-on glass (SOG) is applied to the semiconductor substrate as a planarization layer during the fabrication process.
Spin-on glasses (SOGs) are proprietary liquid solutions containing silicate (purely inorganic SOGs) or siloxane (quasi-inorganic SOGs) based monomers diluted in various kinds of solvents or alcohols. Such solutions are available from Allied Signal Inc., Milpitas, Calif. During coating and curing, the monomers are polymerized by condensation of silanol and ethoxy groups. Water vapour, ethyl alcohol, and other by-products such as ethylene, are released according to the following scheme: ##STR1##
Polymerization of the SOG solution stops when the distance between neighboring silanol groups, or ethoxy groups, becomes too large, or when too much condensation gas such as water vapour, ethyl alcohol, or its decomposition by-products, ethylene, and water vapour, blocks the condensation mechanism. Heating is then needed to eliminate these gases and permit further condensation, densification, and the fabrication of a hard purely inorganic or quasi-inorganic SOG film.
The final density of the obtained SOG films depends on many factors, but it is generally lower than the density of other inorganic or quasi-inorganic glasses deposited by other commonly used techniques such as LPCVD or PECVD. This lower density is due to the large number of pores in the SOG film, which cause high conductance paths between film surface and its bulk.
These pores permit readily adsorbed gas molecules present on SOG film surface to continuously and rapidly diffuse through the bulk of the film and rapidly physically connect to the glass by forming low energy (&lt;0.3 eV) Van der Waals bonds ( . . . ). The gas molecules are rapidly physically absorbed by the SOG film network.
Some gases, such as water vapour, have molecules that can slowly form high energy (&gt;0.5 eV) chemical bonds (.) with the SOG film network by forming a pair of silanol groups. Water vapour molecules are slowly chemically absorbed by the SOG film network as shown below: ##STR2##
This slow chemical absorption of water vapour by the SOG film is particularly efficient if the SOG solution contains phosphorus organometallic molecules, which provide very efficient water vapour gettering due to the presence of phosphorus-oxygen double bounds in the SOG film: ##STR3##
The gettering of water vapour by absorption can be verified by infrared spectroscopy by monitoring the behaviour of these P:O bonds and other bonds in presence of moisture. Infrared spectroscopy shows that readily adsorbed water vapour molecules rapidly diffuse in the pores of the SOG, rapidly become physically absorbed (3350 cm.sup.-1), and slowly become chemically absorbed by --P:O bonds (1280 cm.sup.-1) and --Si.O.Si--bonds (460, 810, 1070 and 1140 cm.sup.-1) to form respectively --P.OH (950 and 3700 cm.sup.-1) and --Si.OH (930 and 3650 cm.sup.-1) groups.
The rapid physical absorption is characterized by a t.sup.1/2 behaviour and the slow chemical absorption is characterized by a t.sup.1/4 behaviour.
The same mechanism is observed for glasses other than spin-on glasses and has been reported for very low temperature, thus very porous, LPCVD and PECVD silicates [See W. A. Pliskin, J. Vac. Sci. Technol., Vol. 14, p. 1064.; J. A. Theil, D. V. Tsu, G. Lucovsky, Journal of Electronic Materials, Vol. 19, No. 3, pp. 209-217].
It is believed that this mechanism is universal and should be observed for any type of porous glass. It is also believed that the lower the film porosity, which implies fewer pores and lower conductance from the film surface to its core, the slower the water vapour channeling, its physical absorption and chemical absorption.
Since the cure temperature of SOG used to smooth glasses in a non etch-back SOG multi-level interconnect process is limited to less than 550.degree. C., and since SOG is more porous than the surrounding LPCVD or PECVD glass, it physically adsorbs more water vapour, as bonded H.sub.2 O, and chemically adsorbs more of it, as .SiOH groups, than its equivalent volume of denser PECVD or LPCVD glass.
If these SOG or LPCVD or PECVD glasses incorporate phosphorus, water vapour can also be chemically absorbed as .POH groups.
If the condensed ethyl alcohol, is not eliminated from the SOG during curing, it can slowly react chemically with the network and form a silanol pair.
The presence of water vapour, ethyl alcohol, and its decomposition by-products in the SOG, as well as in very low temperature LPCVD and PECVD glasses, can cause serious manufacturing problems, such as via poisoning, poor metallization step coverage, dielectric cracking and blistering, as well as reliability problems, such as via corrosion, stress migration, electromigration, transistor threshold voltage instability problems, and hot electron effects, in the finished device.
In the manufacture of multilevel integrated circuits, it is important to ensure complete desorption of adsorbed as well as physically and chemically absorbed water vapour and ethyl alcohol from the SOG just prior to deposition of the second level of interconnect material. This metal based interconnect material covers the top, sidewall and bottom of the SOG exposed via and, in combination with a layer of dense LPCVD or PECVD dielectric that protects the SOG in a non etch-back SOG process, prevents readsorption and reabsorption of water vapour and other ambient gases by the SOG.
An important side benefit of a SOG layer that is completely free of water vapour and ethyl alcohol is that the obtained device then incorporates an integrated gettering material. The dry SOG ensures improved reliability against water vapour penetration up to the device active transistors during highly accelerated stress testing (HAST), temperature and humidity bias testing (THB), other reliability testing, and in the field.
It has been shown that it is possible to ensure desorption of adsorbed, physically absorbed and chemically absorbed water vapor and to leave SOG pores under vacuum before capping the SOG layer by the deposition of dense and protective LPCVD or PECVD dielectric. It has also been shown that the exposure of SOG layer to ambient occurs after via opening and that water vapour readsorption and physical reabsorption occur rapidly while chemical reabsorption occurs more slowly.
Fabrication of semiconductor devices using non etch-back purely inorganic spin-on glass processes typically requires batch type metallization equipment and very short delays between via etching, post via etch photoresist stripping, post stripping inspection, and via metallization itself. This process uses tightly controlled ambient dry boxes for wafer storage if extended periods are needed between these steps; this avoids slow chemical absorption of water vapour by the exposed SOG.
Just prior to entering the metallization equipment, a pre-metallization water vapour desorption step may be performed in an independent vacuum or dry ambient batch system for an extended period of more than about 30 minutes and at a temperature as high as about 450.degree. C. The re-exposure to ambient air causes quick surface readsorption and physical reabsorption of water vapour, while the slow chemical reabsorption of water vapour is prevented due to a too short re-exposure time; the longer the SOG film exposure to ambient air, the more chemically water vapour is absorbed.
An optional in-situ pre-metallization desorption step of adsorbed and physically absorbed water vapour in the load-lock or in the main chamber of the sputtering system may be performed. This is done at relatively low temperature, typically between 250.degree. and 400.degree. C. This batch desorption step is normally not possible in single wafer sputtering equipment, and this explains why single wafer sputtering equipment is normally not used for such a process. If this desorption step is done in the main sputtering chamber, shuttered target sputtering is necessary to ensure cleaning of the targets contaminated by desorption gases. Chemically absorbed species of activation energies exceeding about 2.00 eV are then not eliminated normally. The desorption efficiency is a direct function of the in-line cure cycle on the SOG processor [TAZMO], and the use of standard 250.degree. C. hot plates instead of specially made 350.degree. C. hot plates causes the accumulation of larger quantities of chemically absorbed organic molecules and imposes higher desorption temperature than 400.degree. C.
A sputter etch cycle is typically performed in the main sputtering chamber to sputter clean the undesirable silicon oxide or aluminum oxide thin film covering the silicon or aluminum surface to interconnect with the material to deposit. This step is done at room temperature or at a non controlled and relatively low temperature. The wafer temperature rises during sputter cleaning because of a poor thermal contact to the backing plane. Desorption gases produced by this uncontrolled temperature rise and this glow discharge exposure can cause problems for oxide removal and can in fact contribute to an increase of the oxide thickness. This problem is a very serious one for small diameter vias that are needed for the new generation of devices.
The deposition of selective or blanket tungsten or other interconnect materials by plasma enhanced chemical vapour deposition, PECVD, exhibits the same problem of glow discharge induced desorption since these processes also use glow discharges for the deposition of the interconnect material.